A heat-transfer tube accommodated in a heat exchanger is heated by combustion exhaust gas of a gas burner, whereby a temperature of a fluid flowing inside the heat-transfer tube is raised. There has been known an insertion of a turbulence generator configured to generate turbulence in the fluid into the heat-transfer tube in order to suppress local boiling of the fluid inside the heat-transfer tube or promote heat exchange and increase heat efficiency. (For example, Japanese Unexamined Patent Publications No. 2000-266488 A, No. H11-108458 A, No. H11-51491 A and No. H11-83196 A)
For example, as shown in FIG. 5, there has been known a turbulence generator having a plurality of cut-and-raised pieces 31a, 31b, 31c projecting to both front and back surface sides, by applying cut-and-raising-and-bending work to a flat plate member 3. When the fluid flowing inside the heat-transfer tube collides with the cut-and-raised pieces 31a, 31b, the fluid flows to a tube wall side of the heat-transfer tube, as indicated by solid arrow in FIG. 5. Subsequently, after colliding with the tube wall, the fluid collides with the next cut-and-raised piece 31c. The fluid repeats the above-described flow inside the heat-transfer tube. In accordance with this flow of the fluid, the turbulence of the fluid is promoted, so that the local boiling is efficiently suppressed. Further, in order to reduce pressure loss of the fluid by the cut-and-raised pieces 31a, 31b, 31c, all the cut-and-raised pieces 31a, 31b, 31c of the turbulence generator are inclined in a same direction (a downstream direction) so as not to hinder the flow of the fluid.
However, as described above, when all the cut-and-raised pieces 31a, 31b, 31c are inclined in the same direction, the turbulence generator needs to be inserted into the heat-transfer tube in a specific direction in such a manner that inclination directions of the cut-and-raised pieces 31a, 31b, 31c is consistent with a flow passage direction of the fluid flowing inside the heat-transfer tube, and thus, assemble workability is bad. That is, if the turbulence generator is inserted into the heat-transfer tube in a wrong direction where the flow passage direction of the fluid inside the heat-transfer tube and the inclination directions of the cut-and-raised pieces 31a, 31b, 31c are opposed to each other, the pressure loss with respect to the fluid significantly increases. Further, as indicated by two-dot chain arrow in FIG. 5, after the fluid collides with the cut-and-raised piece 31c projecting to the front surface side of the flat plate member 3, the fluid passes through a cut-and-raised hole 30c. Subsequently, after the fluid collides with the next cut-and-raised piece 31b projecting to the back surface side of the flat plate member 3, the fluid passes through a next cut-and-raised hole 30b. In this manner, if the turbulence generator is inserted into the heat-transfer tube in the wrong direction, the fluid flows in the vicinity of the front and back surfaces of the flat plate member 3. As a result, the fluid gathers to a vertically central portion inside the heat-transfer tube, so that it hardly flows to the tube wall side of the heat-transfer tube. Therefore, the turbulence is not efficiently generated on the tube wall side, and the local boiling occurs or the heat efficiency of the heat exchanger deteriorates.